If the waste water is discharged into the river & if mixes with the river water completely. The following data is available:

i) DO & discharge of Waste water = 3 mg / L & 1.5 m³ / s

ii) DO & discharge of River water = 7.5 mg / L 6.5 m³ / s

Assuming temperature of the mixture as 20 °C. The resulting DO of the mixture is